If like me you want to develop Flex applications with a Design View so you can actually see what you're doing as you code, check out IntelliJ and the Flash Designer UI plugin. It supports Flex 4.8+ and features a design view that runs in an AIR application. For me this is better than Flash Builder's previous design view since it now means I can code on one monitor and see the changes (in real-time too!!) in the design view app on my other monitor - essentially reducing development time.
Would you happen to have any screen shots or anything? Sounds interesting.
We haven't yet made the switch since we have no plans to use the new SDKs just yet. However today I found a way to enable Design View with the new SDKs, fully operational in Flash Builder 4.6. I had to remove the new SDKs from Flash Builder first, then edit the flex-sdk-description.xml file located in the SDK folder, changing the "version" value to "4.6.0". Then I re-added the SDK into Flash Builder and changed my existing project to the new SDK, and hey I had a fully operational Design View. I tried this with Flex SDK 4.9 and it works great. Worth a try if you want to be able to design applications using the latest SDKs.